1. Field of the Invention
This invention relates to a process for electrolytically coloring aluminum or any of its various alloys.
2. Prior Art
For electrolytic coloring of aluminum or aluminum alloy, there has been suggested and practiced extensively a process wherein the basis metal is first anodized to form an oxide film thereon and then electrolyzed in an electrolytic solution containing a metallic salt such as a nickel salt by use of alternating or direct current. The basis metal is colored by electrodeposition of the metal or metal oxide in the pores of the oxide film thereon.
Such a prior art process is generally poor in throwing power, so that particularly in the case of an irregularly shaped workpiece, its protuberant and recessed portions tend to be colored in noticeably different shades. Moreover, since a darker shade is usually produced on the end faces of workpieces, the counter electrode requires intricate masking.
It has also been proposed to make suitably combined use, in the electrolysis of the anodized basis metal in accordance with the above described prior art process, of alternating and direct currents or of other currents similar to or quite dissimilar from such currents in waveform. This practice also has drawbacks such as the complexity of equipment required and the difficulty of control. Another disadvantage arises from the fact that according to such known methods, the degree or depth of coloring of successive workpieces must be controlled by the period of electrolysis. It has been highly difficult to eliminate differences in the coloring degree of the workpieces because of the inevitable personal differences of personnel engaged in color matching operation.